The proposed studies will examine the role of prolactin, estradiol and brain catecholaminergic and LHRH neurons in the age-related transition to irregular estrous cyclicity and eventual complete acyclicity. Experiments, using the 8-16 month old female rat as an experimental model, have been designed to determine which of the following working hypotheses is (are) the most likely explanation(s) for the onset of irregular cyclicity and acyclicity: (1) Prolactin and/or estradiol may become elevated in aging females and feedback to alter the cyclic profile of catecholamine-LHRH dynamics and cyclic gonadotropin release. (2) Age-related changes at the hypothalamic level may change the synthesis, metabolism and/or release of catecholamines and/or LHRH. The spontaneous neural changes result in an acyclic pattern of gonadotropin release. (3) Alterations in the responsiveness of the preoptic area to the positive feedback action of estradiol may be the primary age-related change such that steroids become incapable of stimulating cyclic gonadotropin surges. (4) Changes in pituitary lactotroph sensitivity to dopamine and/or estradiol may occur early during the aging process and lead to changes in prolactin secretion and ultimately to acyclicity. It is possible to test these hypotheses bacause of several recent methodological advances. A microdissection technique used in conjunction with sensitive radioenzymatic and radioimmunoassays for norepinephrine, dopamine and LHRH permit the analysis of changes in these parameters in specific brain structures which are known to be involved in the basal secretion and surge of gonadotropin and prolactin. Catecholamine turnover rates (alpha methyl-para tyrosine method) will be used as an index of neuronal activity. New receptor assays for estradiol and prolactin allow examination of changes in receptor binding in the brain and pituitary with minimal pooling animals for tissue samples. These techniques allow for neuroendocrine analyses to proceed at a greater level of resolution than previously possible such that the questions posed in this proposal may be approached.